pmb.chroot.apk_static.download: fix mirrordir
When selecting the stable channel in "pmbootstrap init", do not attempt to download apk-tools-static from the "edge" mirrordir. This stopped working, because the version in the APKINDEX of v3.12 is different than the one in edge now.
Fixes:
Download http://dl-2.alpinelinux.org/alpine/edge/main/x86_64/apk-tools-static-2.10.5-r1.apk
ERROR: HTTP Error 404: Not Found
Related: https://builds.sr.ht/~postmarketos/job/272760#task-pmbootstrap_build-123
Trivial and important fix, merging without review once CI passes.